\begin{center}
\rowheight=0.8pc
\xytree{
             &                 & \xynode{$x$} \xyconnect{1,-1}"_{\scriptsize$a(y)$}"
					      \xyconnect{1,1}"^{\scriptsize$\overline{a}$}"& \\
             &\xynode{$\bullet$} \xyconnect{1,-1}"_{\scriptsize$b$}" 
			 \xyconnect{1,1}"^{\scriptsize$c$}"&                    
		&\xynode{$z$} \xyconnect{1,0}"^{\scriptsize$d$}"\\
\xynode{$y$} &                 &  \xynode{$\bullet$}        & \xynode{$\bullet$}\xyconnect{1,0}"^{\scriptsize$e$}" \\
             &                 &                    & \xynode{$\bullet$} 
}
\end{center}